Modelling Ion Migration in Bioactive Glasses

Bioactive glasses (bioglasses) are widely used in biomedicine as restorative and regenerative implants, which exploit their ability to bond to hard (bone, teeth) and soft (tendons, ligaments) tissues shortly after exposure to the body physiological environment. This ability reflects a reactive response of the material to the contact with physiological fluids, involving a series of physico-chemical processes, leading to the formation of a layer of bone-like apatite (Ap) on the glass surface within a few hours or days after implantation. The Ap layer provides a strong interface effectively bonding the material and the living tissues: this stable biomaterial-tissue link promotes the integration of the implant and is therefore central for its success. Since the 1980s many studies have led to significant developments in this field: the glass bioactivity, i.e., its ability to bond to bone and/or to induce tissue repair and regeneration, is usually assessed by measuring the rate of Ap formation in-vitro or in-vivo, and the importance of glass composition, particle morphology, surface texture, and thermal treatment is now well established. One major obstacle to further technological progress is that, despite their importance, structure-bioactivity relationships are still largely unknown for bioglasses, mostly due to lack of accurate structural data. The disordered and multicomponent nature of these materials hinders the application of standard experimental probes to access their structure, with the result that prediction and test of compositional effects mostly relies on inefficient and expensive trial-and-error approaches: while the range and level of bioactivity of typical melt-derived compositions has been determined, no rational interpretation of the sharp changes in bioactivity with the composition has been proposed. Any such interpretation requires a detailed knowledge of the atomistic structure, at least of the most common melt-derived bioglasses. Structural investigations of the traditional, melt-derived bioglasses are still highly needed: since the bioactivity level of many melt-derived compositions has been exactly measured, these structural investigations can provide direct insight into structure-activity effects, and the resulting knowledge should be transferable to glasses of different composition and/or obtained through different routes. Atomistic computer simulations can provide a high-resolution picture of structural and dynamical features of these materials, thus supporting a more rational approach to identify the links between the composition, the structure, and the bioactivity of these glasses. As for standard experimental techniques, bioactive glasses represent a significant challenge also to modelling approaches. Our recent computational studies have tackled the bulk structure of bioglasses: by modelling compositions of different, known bioactivity, we identified specific structural features marking bioactive or bio-inactive compositions; these bulk structural data and the corresponding insight obtained represent the essential baseline, upon which further specific investigations can be based. A still largely unexplored field is the diffusive dynamics of Na and Ca cations: their migration within the bulk structure plays a critical role in the bioactive mechanism, because the initial leaching of sodium ions into the physiological solution, and the subsequent release of Ca from the glass are both key steps in the bioactive mechanism. Very few data are available on the Na and Ca transport; the present project aims at investigating the diffusive mechanism of modifier cations in bioglasses, using Molecular Dynamics simulations. The final purpose is to identify possible correlations between the glass composition, the local coordination/structure and the transport of modifier cations, which can be linked to the bioactive properties, and therefore improve our current limited understanding of how these materials work.